1. Technical Field
The present disclosure relates to a memory device, and more particularly, to a NAND type non-volatile memory device and a method for forming the same.
2. Discussion of the Related Art
Non-volatile memory devices retain stored data even when no power is supplied to it. One example of a non-volatile memory device is a flash memory device. The flash memory device writes and erases data using electric signals. The flash memory device may be classified as a NOR type flash memory device or a NAND type flash memory device. Since the NOR type flash memory device is capable of performing high speed random access, it is widely used in devices that require high speed operation. Since the NAND type flash memory device has an excellent programming and erasing speed and can be easily realized in high integration, it is widely utilized in high-capacity storage units.
The NAND type flash memory device includes a plurality of cell strings. Each cell string includes a plurality of unit cells connected in series. A string select transistor and a ground select transistor are connected to two sides of each of the unit cells, respectively. The unit cells and select transistors are formed on an active region of a semiconductor substrate. The NAND type flash memory device applies a predetermined bias into the active region. Accordingly, the NAND type flash memory device has a structure in which a bias is applied into a well.
A structure for applying a well bias is discussed in Korean Patent Publication (NO. 1999-31469). According to the structure, a region where a well contact is formed is larger than a neighboring region where a cell string is formed. Additionally, to form the well contact, a portion of gate lines may be cut. Accordingly, continuity and/or repeatability of patterns become(s) irregular due to the well contact in the semiconductor substrate. Due to the irregularity of continuity and/or repeatability in the patterns, a photolithography process becomes unstable. For example, because of a difference in an amount of scattered reflection during exposure due to the discontinuity and non-repeatability of patterns, product defects occur during a photolithography process. Accordingly, other neighboring patterns (e.g., an active region where cells are formed, gate lines and/or select gate lines in cells) where the well contacts are formed may have an irregular form. Consequently, since the distribution of the non-volatile memory cells becomes irregular, it may cause characteristic deterioration and malfunction in the NAND type non-volatile memory device. Since there is a trend towards highly-integrated semiconductor devices, the minimum line width of the device is reduced. Therefore, pattern defects due to the discontinuity and/or non-repeatability in patterns occur more frequently.